1. Field of the Invention
The present invention relates to techniques for coupling optical signals onto a semiconductor chip. More specifically, the present invention relates to a multi-stage technique for coupling optical signals onto a semiconductor chip.
2. Related Art
In order to meet the bandwidth-density demands of future high-performance computing systems, it is useful to be able to multiplex multiple data channels onto a single fiber. One technique for achieving this multiplexing is wavelength division multiplexing (WDM). Unfortunately, an undesirable consequence of WDM is that the optical components needed to couple light comprising of a large number of wavelengths onto a semiconductor chip can often be difficult and costly to implement.
Several types of WDM multiplexing techniques presently exist. Dense WDM (DWDM) uses a narrow wavelength spacing and is typically implemented by modulating data directly onto a highly-stable optical carrier, then combining these carriers into the fiber. The advantage of DWDM is that a large number of channels can be accommodated within a given wavelength band, and hence the highest performance systems use this technique. Coarse WDM (CWDM) uses larger source line widths and is more inexpensive to implement than DWDM. However, CWDM can experience larger temperature dependent wavelength drifts. Hence, there exists a tradeoff between the spacing of the wavelengths and the number of wavelengths that can be accommodated by the optical coupler. Note that time division multiplexing (TDM) techniques can be used to bring data up to the transmission rate.
Furthermore, several optical coupler geometries can be used to couple light from an optical fiber to a semiconductor chip. For example, a tapered waveguide can be used to couple light into a sub-micron sized waveguide on a semiconductor chip with low loss. This provides a large wavelength range, but requires edge-coupling of the fibers and hence does not allow a two-dimensional array of couplers to be formed on the surface of the chip. Furthermore, a tapered waveguide does not allow wafer-scale optical testing of the optical devices because the wafers containing the chips have to be sawed in order to expose the sides of the chip for coupling into the tapered waveguides. Grating couplers can also be used to couple surface-normal, or near surface-normal light from optical fibers into sub-micron waveguides with low loss. However, since grating couplers are typically sensitive to a relatively narrow range of wavelengths, the wavelength bandwidth of the grating coupler is reduced.
Thus, to accomplish efficient optical communication, a technique for coupling substantially surface-normal light into the plane of a semiconductor chip with a large wavelength range is desirable. Additionally, it is desirable to obtain the performance advantages achievable with WDM without the resultant costs associated with DWDM components.